Graphical data input systems have been used for about three decades, and various aspects of the prior art have been the subject of previous patents. There remains a need for a reliable and convenient graphical input system applicable to a large input space, and for which both the input device and the surface on which it moves are completely passive. This would enable, for example, murine real-time capture of a lecturer's writing and drawing on a wall-sized writing surface, with an inexpensive and reliable system.
In the prior art, there are many types of graphical input systems. The most common are "digitizing tablets" or "touch-sensitive screens" used for computer input devices. These make use of various pressure-sensitive or magnetic detection techniques to locate a stylus on an active screen or digitizing surface. Because the active surface areas for all known touch-sensitive surfaces are expensive to manufacture in large sizes (to the inventors knowledge, none are presently available for a "blackboard" sized input area, having height and width dimensions of several meters), the application of this class of approaches for digitizing on large surface areas is not attractive. The manufacturing cost of the touch-sensitive screens for digitization are proportional to some power of the size of the writing surface and therefore suffer from an inherent disadvantage when large input areas are needed. The most pertinent of the recent inventions in this area are those by Yantiv et al (U.S. Pat. Nos. 4,827,084 and 4,827,085). These patents also detail fax-like simultaneous telephone and hand written communications over touch tone telephones. Several other inventions relate to utilization of touch sensitive screen technology. Bloom (U.S. Pat. No. 4,622,437), in spite of the high cost of larger touch sensitive screens, relies upon this technology to digitize input on relatively large data planes.
A second branch of the prior art deal with triangulation using ultra-sonic acoustic waves. This set of approaches is appropriate for digitizing on larger surfaces, and there are several commercially available systems such as those manufactured by Science Accessories Corporation of Stratford, Conn. The acoustic methods have the advantage that they can be extended to digitize in three dimensions. However there remain limitations on accuracy and practical operation of these devices as discussed below. Three embodiments of acoustical triangulation have been described in the prior art: (1) a stylus with acoustic transmitter near the tip with a plurality of stationary receivers, (2) a plurality of stationary transmitters with the stylus being the receiver, and (3) a plurality of stationary transmitters and receivers with acoustical waves being reflected off the moveable element. The first two approaches depend upon using the known difference between the times of transmission and arrival of each burst of ultra-sonic acoustic waves, and an active stylus is needed; thus the stylus requires power and timing information and this has usually been transmitted from a computer via a cord. The third class of ultrasonic devices have a passive, sound-reflecting writing instrument, but this approach has not found many applications due to reliability and other limitations. For all of the acoustic approaches, the finite speed of sound in air (.about.1000 ft/sec) and the variability due to atmospheric changes present practical difficulties which limit the accuracy and digitization frequency of these devices.
This second branch of the prior art is well-described by the following U.S. patents: Whetstone (U.S. Pat. No. 3,838,212), which uses a moveable sparking stylus with stationary receivers. The high voltage spark requires a power cord, and is distracting due to the audible acoustic emission of the spark, and has limited applications due to electromagnetic interference and safety considerations. Davis et. al. (U.S. Pat. No. 4,012,588) employ a sparking stylus in their preferred embodiment, but also describe another embodiment in which the stylus is made cordless by using a stylus which reflects acoustic waves from stationary transmitters to stationary receivers. Others (e.g., Mallicoat, U.S. Pat. No. 4,777,329, Stefik et. al. 4,814,552) employ a cordless stylus where timing data is communicated via electromagnetic waves from a computer to the stylus. Due to the relatively low speed of sound in air, and also due to the directionality of acoustic transmitters and receivers, the acoustic approach encounters limitations on accuracy, frequency of measurements, and reliability. As a consequence, none of the commercially available systems have maximum sample rates of more than 100 (x, y) pairs/second and operate reliably up to sample frequencies of about 50 (x, y) pairs/second, while spatial sensitivity of 0.01 inch can be obtained with careful calibration, accuracy over large surface areas cannot be guaranteed without frequent re-calibration due to atmospheric density variations, and the greater the distance from the emitter to the receivers, the more likely that one or more of the receivers win not hear the signal due to attenuation and directionality of the transmitters and receivers. The use of an active stylus on a power cord, with the corresponding cost, safety, and ease of use implications, is unattractive for many applications.
A third approach exists in the prior art which makes use of optical sensors and laser scanners to locate the stylus. The concept introduced in 1971 by Cooreman (U.S. Pat. No. 3,613,066) is the key historical invention in this area. This patent introduced the idea of using two laser beams to scan a plane and reflect off a stylus to determine two angles and therefrom, the position of the stylus. Cooreman's preferred embodiment used a single laser with a beam splitter to generate the two scanning laser beams where scans were caused by moveable mirrors. Angle encoders were used to measure each mirror's motion and thereby determine the laser beam scan angles. Manufacturing of data input devices based upon Cooreman's invention was not pursued, until the 1980s, most likely due to several un-resolved problems in Cooreman's invention: (i) no provision was made for an accurate pen-up and pen-down detection, and (ii) no practical method was established for generating sufficiently accurate and stable alignment and mirror motion measurements required to implement the preferred embodiment presented by Cooreman.
In U.S. Pat. Nos. 4,642,422 and 4,772,763, Levine and Garwin present enhancements of Cooreman's invention; they introduce a method for calibration and processing of the measured scan angles to determine the digitized coordinates. Their enhancements include a modification of Cooreman's invention whereby a single laser and scanner can be used to determine the stylus position. This embodiment uses reflections off two dihedral mirrored surfaces and two retro-reflecting boundaries. As a consequence of the two dihedral mirrors, the scanned laser beam is reflected such that it encounters the stylus twice per laser beam scan. The stylus coordinates can be determined by finding the scan angles at which the stylus is encountered using means described by Levine and Garwin. Their invention made Cooreman's concept practical, and was especially designed for the case for which the data surface is a computer screen. While U.S. Pat. Nos. 4,642,422 and 4,772,763 represent useful enhancements of Cooreman's invention (U.S. Pat. No. 3,613,066), their embodiment has two key disadvantages as compared to the invention described in this patent: (i) Their sensing and data processing concepts require the real-time computation of the tangents of the scan angles to determine the digitized coordinates, and (ii) their invention requires two precisely manufactured and mounted dihedral mirrors along the border of the data surface. The first disadvantage (i), for a given processor, imposes a computational limit upon the maximum scan frequency. Both the first and second disadvantages affect the cost of the practical implementation of Levine and Garwin's invention. The present invention introduces novel concepts which eliminates these two disadvantages.
In 1987, Lapeyre (U.S. Pat. No, 4,688,933) explored an approach to digitization using two photodetectors arranged such that the detectors can sense light emitted from a light emitting stylus tip. Lapeyre's invention provides a scanner-free detector, but the resulting signal is an angular measurement which requires a corresponding high precision method to make these angular measurements and thereby achieve the desired digitization accuracy. For this reason, Lapeyre's invention is not applicable to high precision digitization over large areas.
There is another well-known means in the prior art for recording and digitizing hand-written information, namely through the use of video cameras to acquire a movie of the hand-writing, and digital image processing to subsequently digitize the desired hand-drawn information visible in the videotape. This means, however, with a single fixed camera, cannot inexpensively acquire high precision digitization of (x, y) needed over large surface areas with existing technology. It is possible, of course, to extract sufficiently accurate information using extensive digital image data processing and video recording(s) made from a moveable camera(s).
In a more recent invention by Gadhok (U.S. Pat. No. 4,732,440), significant advances have been made in resonant laser scanners which make use of ultra-stable flexural pivots with an embedded optical sensor to determine the position of the scanning mirror. Gadhok's invention makes stable resonant scanning feasible with wobbles of less than 10 micro-radians at scan frequencies ranging from 50 to 5000 Hertz. The present invention combines key features of Cooreman's original laser scanning invention (U.S. Pat. No. 3,613,066) with Gadhok's (U.S. Pat . No. 4,732,440 ), and several novel enhancements. In essence, this invention realizes and extends Cooreman's 1971 objectives by enhanced means. The enhancements enable high-precision, high-frequency digitization over very large input surfaces.